Traditional x-ray imaging systems include an x-ray source and a detector array. X-rays are generated by the x-ray source, passed through an object, and are detected by the detector array. Electrical signals generated by the detector array are conditioned to reconstruct an x-ray image of the object.
CT imaging systems include a gantry that rotates at various speeds in order to create a 360° image. The gantry contains an x-ray source having a single focal spot CT tube assembly that generates x-rays across a vacuum gap between a cathode and an anode. In order to generate the x-rays, a large voltage potential is created across the vacuum gap allowing electrons, in the form of an electron beam, to be emitted from the cathode to a single target surface on the anode. In releasing of the electrons, a filament contained within the cathode is heated to incandescence by passing an electric current therethrough. The electrons are accelerated by the high voltage potential and Impinge on the target surface at a single focal spot, whereby they are abruptly slowed down, directed at an impingement angle α of approximately 90°, to emit x-rays through a CT tube window.
Traditionally, scanning widths of an object have been limited due to the feasibly usable maximum angle of the x-ray beam and capabilities of the detector array, which in combination affect quality of a reconstructed image. Typical scanning widths of an imaging tube are approximately 10 mm. The width of the x-ray beam at the detector array is 10 mm and thus the width of the detector array is also 10 mm. With recent developments in CT detector arrays that indicate that the total detector array width or number of slice capability is increasing, limitation of scanning width has become increasingly more dependent upon maximum angle of the x-ray beam. Current CT imaging systems have 16-slice capability, and larger slice capability is foreseeable in the future.
It has been suggested to utilize the current x-ray source with updated larger width detector arrays. A fundamental limit exists when using a single focal spot tube with larger width detector arrays. The larger the width of the detector arrays the more cone-beam artifacts that are produced, causing a reduction in image quality. Another limit associated with single focal spot tubes is that the resolving power of the electron beam decreases from a center ray, extending through the center of the focal spot, towards outer edges of the focal spot. Therefore, detector elements farther away from a center of the focal spot receive a lower resolving power causing poorer image quality for the elements with lower resolving power.
It is also desirable in CT imaging to increase speed of an imaging system without degradation of image quality. CT imaging systems are limited in scanning speed of an image due to the maximum angle of the x-ray beam. With the current scanning angle, for example, only a portion of an organ can be scanned for a single revolution of the gantry, thus requiring multiple rotations and significant amounts of scanning time.
Additionally, in design of an imaging system several other concerns are to be taken into account. One is the desire to mitigate problems associated with conditioning surfaces of a target in preparation for high voltage application.
Another desire is to minimize high voltage instability within the imaging tube. One mechanism for high voltage instability is high vapor pressure, due to gas species such as background gas, surface-absorbed gas, target surface bulk absorbed gas, or track material atoms. These gas species provide ionization targets for incident electron flux producing charged ions. The charged ions and excess electrons produce a low impedance path between high anode and cathode direct current (DC) potentials, which generates “spit” activity. Spit activity can reduce image quality and potentially prevent image reconstruction.
Thus, there exists a need for an improved imaging system that is capable of performing a wide scan of a patient organ or of an object with increased scanning speed while at least maintaining current image quality.